1. Field of the Invention
The present invention relates to an LED backlight driving circuit, more particularly, to an LED backlight driving circuit capable of effectively decreasing signals of electromagnetic interference (EMI), and a liquid crystal display device thereof.
2. Description of the Prior Art
The backlight technique of a liquid crystal display (LCD) develops continuously along with development of relating art. A backlight source of the conventional LCD device applies cold cathode fluorescence lamp (CCFL). However, because of disadvantages such as low color restoration capability, low luminous efficiency, high discharge tension, low discharge property in low temperature and long duration of time for being heated to stable grayscale, a backlight source technique applying an LED backlight source has been exploited. In an LCD device, the LED backlight source and an LCD display panel are set up in opposition, so that the LED backlight source supplies a light source to the LCD display panel. The LED backlight source comprises at least a string of LEDs, and every string of LEDs comprises multiple LEDs.
FIG. 1 is a driving circuit of a conventional LED backlight source applied in the LCD device. As FIG. 1 indicates, the driving circuit of the LED backlight source comprises a voltage booster circuit 110, an LED unit 120 and a constant current driving integrated chip (IC) 130. The voltage booster circuit 110 is controlled by the constant current driving IC 130, so that input voltage is conversed to needed output voltage and hence supplied to the LED unit 120. The constant current driving IC 130 outputs a driving signal S to control on/off state of a MOS transistor Q in the voltage booster circuit 110. When the MOS transistor Q turns on, an input voltage Vin exerts on the two ends of an inductance L, causing linear increase of electric current through the inductance L. Due to the limit of electric current the inductance L can bear, however, the duration of time for which the MOS transistor Q turns on in a time cycle must be limited too. In addition, because the output voltage swing needed to light up the LED unit 120 decides the duty cycle of the driving signal S, the frequency of the driving signal S will be as high as between 100 kHz-200 kHz.
Electromagnetic Interference (EMI) means the interference due to interactions between electromagnetic waves and electronic components, comprising two types: conducted interference and radiated interference. Conducted interference means coupling (interfering) signals of one electric network to another electric network through conducted medium. Radiated interference means coupling (interfering) signals of interfering sources to another electric network through space. In a high-speed PCB and a system design, high-frequency signal lines, pins of integrated circuits, various types of socket connectors are all potential antenna characteristic interfering sources, capable of radiating electronic waves and interfering operations of other systems or other subsystems in the system.
In a large size LCD panel, a backlight source needs multiple strings of LEDs parallelly connected with each other. Because a single voltage booster circuit can only provides low electric current, multiple voltage booster circuits have to operate simultaneously in order to drive the backlight source. Conventionally, turns on and turns off of MOS transistors in multiple voltage booster circuits are both controlled by an identical driving signal from one constant current driving IC. Because of relatively high frequency of driving signals, the superposition of multiple high-speed driving signals of the same frequency will result in a relatively strong harmonic wave where frequency doubling exists, causing relatively strong EMI, which will severely interfere the LED driving circuit and the LCD device thereof.